What to do with unassigned phone numbers

Hi, we have some unassigned phone numbers that ported over from our previous provider; we didn't know what to do with them at the time. The prefixes and line numbers do not coincide with our actual company number and so don't want to use them. Can we do anything with these numbers other than delete them? Can they be converted to match the desired prefix/line number sequences if the prefixes/line numbers are available externally? If so, what is the process?